1. Field of the Invention
Embodiments of the present invention relate to the evaporation of stagnant liquids under the effect of air streams and at variable temperature. This phenomenon covers a large number of practical situations, such as: evaporation in the open air, under the effect of wind, of static water surfaces, having areas of various sizes, such as, for example, ponds or swimming pools, manufacturing methods involving a vaporization process or the saturation of a gaseous phase; and evaporation of pools of liquid, either in the open air, or inside a ventilated enclosure. Such situation may correspond to leakage or to accidental spillage of liquid, the vapors of which present risks in terms of toxicity, fire, explosion or air pollution.
2. Description of the Prior Art
In thermal engineering, this very general phenomenon is designated by the expression “forced-convection vaporization”, it being known that such a vaporization process that takes place predominantly under forced convection is very often accompanied by effects of natural-convection vaporization and of diffusion vaporization, which are the only phenomena present in an atmosphere that is completely still.
Knowledge of vaporization kinetics at the surfaces of pools of liquids is a growing need in process engineering and in the context of making industrial facilities safer. In this field, the regulations relating to the risks of fire and explosion associated with leakage of flammable substances in the form not only of gas clouds but also of pools of volatile liquids are becoming generally more stringent worldwide. Unfortunately, the current control of combustion phenomena does not make it possible, in real situations, to determine with sufficient accuracy and certainty the criteria for the transition from normal combustion conditions to deflagration conditions, or even to detonation conditions. In fact, the conditions that govern the development of a shock wave within structures as complex as those existing in industrial facilities, and under widely varying operating conditions, are not sufficiently controlled. Because of the lack of reliable predictive models, regulations remain highly restrictive, assuming that a risk of explosion exists whenever a pocket of gas or vapor appears and might ignite, i.e. whenever the concentration of the flammable substance in the pocket lies in the range extending from its lower flammability limit (LFL) to its upper flammability limit (UFL).
Methods have been developed in attempts to determine experimentally the vaporization kinetics of a flammable liquid under controlled conditions. For example, mention can be made of Patent FR 2 694 092 that relates to a method of measuring the rate of evaporation and the ignition time of a liquid fuel. In that method, a drop of fuel is formed and is placed in a surrounding gaseous environment; an image of the drop is taken on a plane optical receiver; the area of the image is measured over time; the area of the volume of the drop is calculated on the basis of the area of the image; and the temperature of the surrounding gaseous environment is measured simultaneously. That patent thus focuses on a method of monitoring how the volume of a drop of flammable substance in a surrounding gaseous environment varies over time, that variation being determined by means of an optical device.
Patent EP 1 610 125 describes a method of determining the vaporization properties of liquid fuels, for motor vehicle engines, that method having the particularity of using an electric heater element, immersed at least partially in the liquid, so as to conduct heat from the heater element to the liquid constituting the fuel. The invention described in that patent thus heats the fuel by means of heat transfer by conduction.
The methods described in the state of the art make it possible, experimentally, to approach the phenomenon of vaporization of a flammable liquid, but under specific conditions that do not necessarily reflect the real conditions encountered in industrial facilities. It then becomes difficult, or even dangerous, for the results obtained using those methods to be applied directly to real industrial situations, which present conditions that are quite remote from the experimental conditions that prevail in said methods. The vaporization kinetics of a liquid depend not only on the properties of said liquid, such as, for example, the composition and the specific volatility of each component, but also, and to a considerable extent, on the “aerothermal” conditions of the gaseous environment, which is often constituted by air. The term “aerothermal” as used in this document covers not only thermal conditions, such as temperature field, and air-flow conditions, such as velocity field and flow regime, characterized by the Reynolds number, but also the degree of vapor saturation of the gas, in particular its humidity, when the vapor is water vapor.